Monazite, Chevkinite-Perrierite and Xenotime in Martian Breccia

46th Lunar and Planetary Science Conference (2015)
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MONAZITE, CHEVKINITE-PERRIERITE AND XENOTIME IN MARTIAN BRECCIA METEORITE
NWA 7034. Yang Liu1 and Chi Ma2. 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
91109, USA. 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA
91125, USA. (Email: [email protected]).
Introduction: Martian breccias NWA 7034 and its
pairings (NWA 7475, 7533, 7906, 7907, 8114) are
peculiar in many aspects. They consist mainly of Naand K-rich feldspars and pyroxenes with minor magnetite (+ maghemite + rutile exsolution), pyrite, zircon
and phosphates [1-6]. The U-Pb dates from zircon and
apatite suggest a formation age of 4.428 Ga and a resetting age of ~1 Ga [2, 4]. Moreover, the bulk analysis
of NWA 7034 also suggested that this sample is water
rich (up to 6000 ppm) and the step-heating released
water at low temperature contains oxygen isotope
compositions similar to Martian meteorites [1, 6].
In the 8th International Mars Conference, we reported finding of monazite in NWA 7034, representing the
first of such rare-earth minerals in a Martian rock [7].
In order to examine the textural association of monazite in NWA 7034, we examined 3 additional sections,
2 of which are from different pieces. During this
search, we recognize the presence of xenotime, and
chevkinite-perrierite group minerals in NWA 7034, in
addition to the common occurrence of monazite in apatite. These first findings of rare-earth minerals in NWA
7034 suggest the presence of these minerals in Martian
crust, either by formation in alkali-rich igneous systems
or by hydrothermal activities. Both formational mechanisms on Earth involve water-rich fluids.
Results: Owing to the small sizes, the compositions reported here were estimated based on SEM-EDS
measurements using Oxford internal standards and the
XPP matrix correction procedure. Electron microprobe
analysis of these minerals is on-going.
Monazite-(Ce), monazite-(Nd) and chevkiniteperrierite-(Nd)-bearing lithic clast (Fig. 1). This clast
consists of feldspar minerals, including plagioclase
series of variable Na contents, which inter-grow with
alkali-feldspar of compositions up to ~Or75Ab25. Augite (~En50Wo45) is next abundant mineral. Minor to
trace minerals include apatite, magnetite and pyrite
(with alterations), and zircon. The bulk SiO2 content
estimated from constituent minerals places this clast
near the andesitic-dacitic boundary.
The monazite and chevkinite-perrierite grains occur
in a small region (~10 μm by 10 μm), which consists of
a few subhedral monazite and chevkinite-perrierite
grains in a fine mixture of silicates and tiny P-rich and
REE-rich minerals (also monazite?). The monazite
contains 3-4 wt% Y2O3, 3-4 wt% La2O3, ~22 wt%
Ce2O3, ~29 wt% Nd2O3, and 2 wt% Sm2O3 with no
detectable Th. The chevkinite-perrierite grains are with
~4 wt% La2O3, >18 wt% Ce2O3, and >20 wt% Nd2O3.
Figure 1. A lithic clast in NWA 7034 with monazite(Ce) & -(Nd) and chevkinite-perrierite-(Nd) grains.
Aug: augite; plag: plagioclase; K-spar: alkali feldspar; FeO: magnetite.
Figure 2. Monazite-(Ce) inside a chlorapatite in NWA
7034.
46th Lunar and Planetary Science Conference (2015)
Monazite-(Ce) in apatite (Fig. 2). Monazite occurs
as inclusions in chlorapatite grains (mono-mineralic
clasts) in three sections investigated. Compared to
monazite in Fig. 1, these monazite are more rich in Ce
than Nd, and also contains more Y and La with Y2O3
(6-10 wt%) and La2O3 (7-11 wt%).
Xenotime(Y)-bearing lithic clast (Fig. 3). A few
xenotime grains were observed in a clast that mainly
consists of ilmenite and pyrite, which show significant
alteration. The clast also contains abundant zircon
grains. Xenotime was found along grain boundaries,
and contains ~4 wt% of Gd2O3 and 5 wt% Dy2O3.
Previous occurrences in lunar rocks and meteorites: Combining with our report in the 8th International
Mars conference [7], we present the first report of
multiple rare earth minerals (monazite, xenotime, and
chevkinite-perrierite) in a martian rock (NWA 7034).
To date, monazite has only been observed in three occurrences: inclusion in hedenbergite in Apollo 11 basalt 10047 [8], intergrown with whitlockite in Apollo 17
melt breccia 76503 [9], and in a howardite Yamato
7308 [10]. Monazite composition in Yamato 7308 was
not reported. Compared to the monazite grains in
NWA 7034, those in lunar rocks are higher in La2O3
(13-16.5 wt%) and Ce2O3 (27-30 wt%), but lower in
Nd2O3 (12-15 wt%). Chevkinite-perrierite in NWA
7034 is the second such finding in addition to the report of Ce-rich ones in a lunar basalt [11]. However,
xenotime in NWA 7034 is the first occurrence in extraterrestrial samples.
Implications: Monazite in NWA 7034 differs from
terrestrial metamorphic ones in their lower La and
higher Nd [12]. The higher Y and La contents in monazite inclusions in apatites are consistent with igneous
monazite on Earth. However, a detailed compilation of
terrestrial monazite chemistry, particularly those in
more mafic rocks, is lacking [12-14].
The lithic clast with monazite-(Ce), monazite-(Nd)
and chevkinite-perrierite-(Nd) in NWA 7034 differs
from those in lunar samples. In lunar case, these minerals are the very-late-stage phases after the rock went
through extreme crystallization. They are typically
found in mesostasis with Fe-silicates, silica phases, and
apatite. The lithic clast in NWA 7034 has at best a
dacitic bulk composition, and does not contain silica
phases or Fe-silicates as in lunar samples. Thus, the
texture and composition of this clast suggests a possible fluid-induced formation.
The chemistry of xenotime in NWA 7034 is similar
to metamorphic ones on Earth [12]. The appearance of
xenotime and many small zircons in the ilmenitepyrite-rich clast is also the most intriguing. This type of
lithology is atypical for an igneous origin, but is possibly similar to hydrothermal deposits on Earth [13, 14].
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In conclusion, NWA 7034 is a breccia representing
martian crust [1-2], and the different textural associations of rare-earth minerals suggest diverse crustal environments on Mars, some of which may require a hydrothermal environment. As suggested by Herd et al.
[15], finding monazite in a crustal sample offers a possible explanation for LREE depletion in shergottites.
Figure 3. Xenotime-(Y) in a clast in NWA 7034, consisting mainly of ilmenite and pyrite with extensive
alteration to ferric oxide hydroxide. The clast also
contains many small zircon (Zir) grains.
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